KYIV, July 28 (Reuters) - Ukraine stepped up its drive to retake its Russian-controlled south by trying to bomb and isolate Russian troops in hard-to-resupply areas, but it said on Thursday (Jul 28) it saw evidence that Moscow was redeploying its forces to defend the territory.

In messages to mark the annual Day of Ukrainian Statehood, President Volodymyr Zelenskyy congratulated Ukrainians and sounded defiant.

"We will not give up. We will not be intimidated. Ukraine is an independent, free, indivisible state. And it will always be so," he wrote on Telegram.

After Russia and Ukraine struck a deal last week to unblock grain exports from Black Sea ports, UN aid chief Martin Griffiths said he was hopeful the first shipment of grain from a Ukrainian Black Sea port could take place as early as Friday.

He said "crucial" details for the safe passage of vessels were still being worked out, however, adding that "the devil was in the details".

Allowing safe passage for grain shipments from Ukrainian ports should ease shortages that have left tens of millions of people around the world facing soaring food prices and hunger.

Ukraine said on Thursday its planes had struck five Russian strongholds around the city of Kherson and another nearby city.

The southern Kherson region, which borders Russian-annexed Crimea, fell to Russian forces soon after they began what Moscow calls "a special military operation" on Feb 24. Ukraine describes Russia's actions as an imperial-style war of conquest.

British military intelligence, which helps Ukraine, said it was likely that Ukrainian forces had also established a bridgehead south of a river which runs along the region's northern border. "Ukraine's counter-offensive in Kherson is gathering momentum," it said in a statement.

Ukraine says it has retaken some small settlements on the region's northern edge in recent weeks as it tries to push Russian forces back, a potential prelude to what Kyiv has billed as a major counter-offensive to retake the south.

Russia said on Thursday it was unfazed, with the defence ministry saying its planes had attacked a Ukrainian infantry brigade in the far north of Kherson region and killed more than 130 of its soldiers in the last 24 hours.

Kirill Stremousov, the deputy head of the Russian-appointed military-civilian administration running the Kherson region, has also dismissed Western and Ukrainian assessments of the battlefield situation.